Update our range `assume`s to the format that LLVM prefers
I found out in https://github.com/llvm/llvm-project/issues/123278#issuecomment-2597440158 that the way I started emitting the `assume`s in #109993 was suboptimal, and as seen in that LLVM issue the way we're doing it -- with two `assume`s sometimes -- can at times lead to CVP/SCCP not realize what's happening because one of them turns into a `ne` instead of conveying a range.
So this updates how it's emitted from
```
assume( x >= LOW );
assume( x <= HIGH );
```
or
```
// (for ranges that wrap the range)
assume( (x <= LOW) | (x >= HIGH) );
```
to
```
assume( (x - LOW) <= (HIGH - LOW) );
```
so that we don't need multiple `icmp`s nor multiple `assume`s for a single value, and both wrappping and non-wrapping ranges emit the same shape.
(And we don't bother emitting the subtraction if `LOW` is zero, since that's trivial for us to check too.)
One extension worth noting is the use of revisions as custom prefixes for
FileCheck. If your codegen test has different behavior based on the chosen
target or different compiler flags that you want to exercise, you can use a
revisions annotation, like so:
After specifying those variations, you can write different expected, or
explicitly unexpected output by using <prefix>-SAME: and <prefix>-NOT:,
like so:
b2728d5426